1. Field of the Invention
This invention relates to a laser marker and a method of laser marking which use a liquid crystal device as a pattern mask by which pattern information is applied to an object to be worked in order to print the same.
2. Description of the Prior Art
Generally, an object to be worked, for example, an electronic part is printed with such characters as its product number, manufacture date and name with a view of demonstrating its source. Conventionally, a pattern mask for this purpose is prepared by etching a metal sheet to form therein through-holes corresponding to a character standing for pattern information. After the pattern mask is applied on the electronic part and a paint is then coated on the pattern mask, the pattern mask is removed so that the character can be transferred to the electronic part. According to this conventional method, the pattern mask must be exchanged adaptively for different kinds of electronic parts and disadvantageously, the time-consuming exchange work is required and many kinds of pattern masks have to be prepared.
As a countermeasure, a laser marker has been proposed as disclosed in JP-A No. 60-174671. In the laser marker, a liquid crystal device used as a pattern mask is interposed between a laser ascillator and an object to be worked, a voltage representative of a character to be printed on the object is applied to a display portion of the liquid crystal device and a laser beam is then irradiated on the liquid crystal device, so that only the laser beam which has passed through the character can pass through a polarizer plate and irradiate the object to print the character thereon.
However, the resulting print is sometimes sharp and sometimes dim. The cause of this incomplete print has been studied thoroughly by the inventors of the present application and elucidated as will be described below.
FIGS. 1A and 1B illustrate a liquid crystal device 13 in use. Particularly, when a voltage of a power supply 29 is not applied as shown at FIG. 1A, liquid crystal molecules are twisted between electrodes 13A, as depicted at reference numeral 13C. Accordingly, a linearly polarized (for example, P polarized in direction y) incident laser beam 12A is converted into a linearly polarized output laser beam 12B whose plane of polarization is 90.degree. rotated (direction z) with respect to that of the incident laser beam 12A.
As shown FIG. 1B, when a liquid crystal device power switch 25 is turned on to apply the voltage across the electrodes 13A, the liquid crystal molecules are oriented in the direction of electric field E to take the operational state, as depicted at reference numeral 13B. Under this condition, the incident laser beam 12A can pass through the liquid crystal device 13 without having its plane of polarization rotated.
The liquid crystal molecules take a response time to reach the operational state following the closure of the power switch 25 or to recover the original twisted state from the operational state and the response time is about 0.2 to 0.5 seconds, though depending on the kind of the liquid crystal molecules. Accordingly, if the incident laser beam 12A is irradiated over the duration of the response time, then the output laser beam 12A is elliptically polarized. The elliptically polarized laser beam is partly shielded by the succeeding polarizer plate, with the result that a laser beam leaving the polarizer plate is decreased in quantity of light as compared to that of the incident laser beam 12A and fails to print a sharp character on an object to be worked.